Monitors have been increasingly used in photorealistic color production and prepress systems. Accordingly, it is important that any luminance variations across the face of the monitor be corrected in order to insure color accuracy and predictability. If luminance differences exist between color channels, then color purity will be compromised.
Luminance variations can be introduced into a monitor by a number of ways including, but not limited to, aging, shock, non-uniformities in phosphor deposition, shadow mask imperfections or thermal changes, differences in the beam energies of the different electron grins, and the position of the monitor relative to earth's magnetic field. The variations so introduced must be compensated for and corrected to maintain high color purity and uniformity.
One possible solution is to manufacture monitors to tight luminance and color purity specifications. However, such techniques increase their cost.
Existing systems adjust luminance by the addition of expensive, cumbersome, and complex external hardware that intercepts and modifies the video signal in the analog domain before it reaches the monitor.
European Patent Application Publication No. 514 025 A2 by Rasterops Corporation, published Nov. 19, 1992, discloses the use of a video normalizer to correct irregularities in video display monitor screens. A photo sensor detects the light output at various points on the monitor. These output values are converted to digital signals and processed to compute correction values. The correction values are then provided to a separate frame buffer that is external to the computer having a memory location for each pixel on the monitor display. A correction circuit converts these digital signals into analog correction signals which in turn either skew the output signals of the video color processing board or control transconductance amplifiers connected between the video color processing board and the RGB inputs of the monitor. This method has some problems in terms of speed. Applying correction signals in the analog domain or using them to control transconductance amplifiers causes an undesirable delay in processing when individual correction values need to be applied to the individual color components of each pixel. Also, this method provides monitor luminance correction (ie. the same correction to each color component), instead of dynamic purity correction which involves individual correction for each color component.
The present inventor has recognized that a preferable system would be able to rapidly calculate and apply smoothly varying correction signals in the digital domain on a pixel by pixel basis, for each color component, and that it would be advantageous for such a system to continuously calculate correction values for each pixel in real time (at the display dot clock rate) to avoid having to continuously store and retrieve these values. However, until the present invention, it had not been known how to overcome the limitations of the described prior art, or how to implement the described improvements over the prior art.